This invention relates to a multi-stage ATM switch and to a method for switching ATM cells in a multi-stage ATM switch.
In an asynchronous transfer mode (ATM) network, data is transferred in ATM cells. Each ATM cell has a payload data field and a destination field. The destination field includes information on the destination for the cell so that the ATM cell can be properly routed through an ATM switch. As will be appreciated by those skilled in the art, ATM cells generally propagate asynchronously through an ATM network, but are synchronously handled in switches in the network.
Multi-stage ATM switches having an inlet (or first) stage, a core (or second) stage, and an outlet (or third) stage are known. The outlet stage terminates in the outlet ports of the switch. Each stage is typically implemented with a plurality of fabrics to increase the switch size.
In one known multi-stage switch, the destination field of ATM cells routed through the switch specify (i) a destination core stage fabric, (ii) a destination outlet stage fabric, and (iii) a destination outlet port of the destination outlet stage fabric. To ensure that the order of cells is preserved through the switch, cells incoming to a given channel of an inlet stage fabric are always routed through the same core stage fabric. Where a high number of cells arrive on any one channel of an inlet fabric, the ATM switch can become congested.
In another known multi-stage switch, which is described in an article by M. Beshai and E. Munter entitled "Multi-Tera-bit/s Switch based on Burst Transfer and Independent Shared Buffers", Globecom. Singapore, November 1995, the three stages are rotator-linked and, in a straight-forward implementation, comprise an equal number of inlet, core, and outlet stage fabrics. ATM cells routed through the switch have a destination field specifying (i) a destination outlet stage fabric and (ii) a destination outlet port of the destination outlet stage fabric. Each inlet stage fabric has a plurality of queues, one representing each of the outlet stage fabrics. A cell migrating to a given inlet stage fabric is placed into a queue in the inlet stage fabric based on the destination outlet stage fabric identified in the destination field of the ATM cell. In a given connection period, the inlet stage fabrics are connected to the core stage fabrics by an inlet rotator so that, for any given core stage fabric, there is one inlet stage fabric connected to it; in the next connection period, each inlet stage fabric is connected to a different core stage fabric by the inlet rotator. In this way, during an inlet rotator cycle, each inlet stage fabric visits each core stage fabric. Similarly, in a given connection period, the core stage fabrics are connected to the outlet stage fabrics by an outlet rotator on a rotating basis such that during an outlet rotator cycle, each core stage fabric visits each outlet stage fabric.
During a given connection period, each inlet stage fabric transfers a burst of cells to the core stage fabric to which it is connected, up to a pre-defined maximum number. The cells in the burst can be destined to different outlet stage fabrics. Each core stage fabric has a number of fixed length buffers, one representative of each outlet stage fabric. The cells in the burst are placed into these buffers based on their destination outlet stage fabric. However, since the buffers are of a fixed length, prior to the burst the core stage fabric indicates to the inlet stage fabric the maximum permissible number of cells from each queue which may be part of the burst.
During the given connection period, each core stage fabric is connected to one outlet stage fabric by the outlet rotator and the core stage fabric transfers the entire contents of its buffer for that outlet stage fabric to the outlet stage fabric. The maximum length of the buffer is chosen so that there is time during the connection period to exhaust the buffer.
It is necessary that the order of cells in a given queue of any one inlet fabric be preserved since these cells could all relate to one connected call. Such cell order is preserved by having each core fabric visited sequentially by inlet fabrics and by having each core fabric visit the outlet fabrics sequentially and exhaust the buffer representing the outlet fabric visited.
Cells arriving at an outlet fabric are placed into queues representing the outlet ports to which the queues are destined.
Such a switch requires control communication between the inlet and control stages.
This invention seeks to overcome drawbacks of known multi-stage ATM switches.